1. Field of the Invention
The present invention relates to an antenna apparatus and a method for mounting an antenna, and more particularly, to mounting a planar antenna, which includes a radiating element and a ground plate, on a vehicle.
2. Description of Related Art
A planar antenna has a radiating element and a ground plate, and is used as a vehicle-mounted antenna. When the planar antenna is installed in a vehicle, the ground plate is mounted on the body of the vehicle.
Referring to FIG. 6A, a planar antenna 61 has a radiating element 62 and a ground plate 63. The radiating element 62 is connected to an inner conductor 66 of a coaxial cable 65 via a feeder 64. The ground plate 63 is connected to an outer conductor 67 of the coaxial cable 65. The ground plate 63 of the planar antenna 61 is mounted directly on the body 69 of a vehicle 68.
Referring to FIG. 6B, in such an antenna 61, electric lines of force, formed by electric fields, start perpendicularly on the radiating element 62, and end perpendicularly on the body 69 away from the planar antenna 61 (shown by solid line arrows E). This is because the electric lines begin and end perpendicularly to the charged surface, and the body 69 has the same electric potential as the ground plate 63. Accordingly, the electric fields in the vertical direction are reduced when the antenna 61 is mounted on the body 69.
Since antenna directivity is formed perpendicularly to the direction of the electric lines, directivity of the antenna 61 is shifted from a horizontal direction toward a vertical direction (shown by dotted line arrow P). As a result, the antenna 61 does not have an appropriate transmission gain in the horizontal direction.